Three-dimensional spatial modelling of nuclear de-excitation lines
Galactic cosmic rays (GCRs) are thought to originate from shock acceleration in supernova remnants, but interstellar ionization rates suggest an additional low-energy cosmic-ray (LECR) component. LECRs excite nuclei in the ISM, producing MeV gamma-ray line emission through subsequent de-excitation. Observations of this emission can constrain the LECR flux, morphology, and acceleration mechanisms. We construct all-sky maps of Galactic de-excitation emission using three-dimensional (3D) CR distribution models. By comparing them with the Galactic diffuse gamma-ray emission, we assess the detectability of the de-excitation line components against the underlying continuum. We calculate the emission by combining 3D CR distributions from GALPROP and gamma-ray production cross sections from TALYS. We also model an additional LECR component motivated by the high ionization rates measured in the ISM. The predicted de-excitation emission is relatively weak when integrated over the broad regions commonly adopted in MeV gamma-ray analyses ($|l|\leq30^{\circ}$ and $|b|\leq15^{\circ}$), contributing only $\lesssim 1\%$ above the leptonic continuum even considering the additional LECR component. The all-sky maps reveal that, in the standard scenario, the de-excitation-to-leptonic flux ratio remains low over the entire sky, reaching at most ${\sim}20\%$ toward the Galactic anticenter. In contrast, models including an additional LECR component exhibit a pronounced enhancement toward the GC, where the ratio exceeds $100\%$, indicating favourable prospects for detection through spatially resolved analyses. The 6.1-to-4.4 MeV intensity ratio is particularly sensitive to the maximum energy of the injected LECR spectrum, varying by up to ${\sim} 30\%$. Future MeV gamma-ray observatories could thus identify a distinct LECR population and constrain its acceleration and transport.